1. Field of the Invention
The present invention relates to a stage having an electrostatic chuck, used for attracting and holding a to-be-processed substrate such as a semiconductor wafer during a process such as etching or film formation for the substrate, and the plasma processing apparatus employing such a stage.
2. Description of the Related Art
In a processing apparatus such as a plasma etching apparatus for processing semiconductor wafer one by one, a susceptor which serves as a lower electrode is provided in its process chamber which can be set to a vacuum state. A wafer, to-be-processed substrate, is placed on and fixed to the susceptor, and then subjected to the process. There are mainly two commonly-used ways of fixing a wafer to a susceptor, i.e. the mechanical supporting means such as clamp, and the electrostatic chuck for attracting a wafer by means of an electrostatic attractive force.
U.S. Pat. No. 4,771,730, issued on Sep. 20, 1988, discloses an electrostatic chuck provided on a susceptor or a table for holding a to-be-processed substrate. The electrostatic chuck includes two dielectric layers, and an electrode interposed therebetween. The electrode is connected to the positive terminal of an external direct current (DC) power supply, and the negative terminal of the direct current power supply is grounded. A heat conductive gas is supplied between the wafer and the upper dielectric layer. The US Patent discloses a structure in which an Al.sub.2 O.sub.3 layer is used as a dielectric layer, in FIG. 2, and a structure in which a polyimide sheet is used as a dielectric layer, in FIG. 3.
When plasma is generated in the process chamber, the semiconductor wafer placed on the electrostatic chuck is grounded via the plasma and the upper electrode. With this structure, if a positive potential is applied to the electrode of the electrostatic chuck from the DC power supply, the wafer will have a negative potential, and the upper dielectric layer is polarized, with its upper surface having a positive potential. Therefore, an electrostatic attractive force is generated between the wafer and the dielectric layer, and due to the attractive force, the wafer is attracted and held on the susceptor.
However, with such an electrostatic chuck, attractive force remains (to be called as "residual attractive force" hereinafter) between the wafer and the chuck even if the electrode of the electrostatic chuck and the power supply are disconnected after the process. In this state, when the wafer is lifted with pusher pins built in the susceptor, a large force is applied to the wafer locally, thereby damaging or displacing the wafer. To solve this problem, there have been proposed several methods of removing residual electric charge on the rear surface of the wafer or the surface of the chuck, as shown in application Ser. No. 08/017,379 now abandoned, filed on Feb. 12, 1993.
In addition, since a dielectric sheet of polyimide is likely to have a dielectric breakdown when it is too thin, the sheet is generally formed to have a thickness of about 50 .mu.m. In accordance with such a thickness, the voltage of the DC power supply should be set high in order to obtain a strong attractive force. However, if a high voltage is involved, an unnecessary discharge is likely to occur, or the cost of the power supply is raised.